Answer: Rosenberg and Trevathan discovered birth patterns in primates and non human primates. They compared the birth patterns of human, gorilla, gibbon, monkey to understand the similarity and differences among them.
In case of monkey and gibbon the dimension of the neonatal cranium have very minute difference to that of their mother's pelvis. When we talk about great apes like gorilla and pan the size of pelvis is large as compared to the cranium size of their neonates and the pelvic inlet is spacious. In terms of human being the cranium size of the neonates is longer than the anterior and posterior dimensions of the pelvic inlet which requires the head to pass through the inlet facing transversely.